Here are some thoughts. I didn't want a speaker grill to keep things a little inconspicuous. The flush mount was nice that CSTMIZR did. I got a grill to use, but it was from a JL sub and didn't have a mounting ring. However, a mounting ring will just be a regular mount and not flush, or subtle.

I also considered re-doing the whole deck in grill cloth. Cut the hole, then redo the whole deck so it is invisible. That is a lot of work. First the deck cloth is nice already. To add cloth it has to be thin. Then you have to get it under and fitted with all the holes and brake light and such and make sure it all fits. (why you would need to use thin cloth) Then you would have to glue that and be sure to get the vent opening very tight so it would not flutter with the bass. Lastly, if anything ever happened, or you put a hole in it, you would be looking at redoing the whole deck. So I passed.

I tried to use my grill in a flush mount, but it did not work. My grill had tapered edges and would not stay. Plan B.

I got some speaker grill cloth from an audio store. They had some pieces laying around they gave me. They also had a sheet they would have sold me if I wanted to do the whole deck.

I did just as CSTMIZR explained. I cut a small hole in the deck. I peeled back the deck cloth. It wasn't as thin and fragile as I thought. It is like 1/8 inch felt type stuff. However, you do want to peel slow and carefully. Be sure to get it all peeling off the backing so you don't tear it off. I kept peeling the cloth back and cutting my hole bigger till it got as big as my grill. I just used a utility knife. Much better faster ways, but I didn't have the tools. Then I realized my grill would not work.

So I had a 8" hole, with the cloth peeled back like mentioned. I used the spray adhesive stuff to glue the pieces around the edge of the opening. Be sure to pull tight so it will be flat. And just like CSTMIZER said, cut slits but not to the edge, or you will see it. The cloth stretches so just cut about half way.

I then decided to glue the grill cloth on the underside of the deck. I used regular spray adhesive to do this and it stuck good. I was worried it would not take to the Styrofoam backing. If you do this, be sure to get some adhesive that will stick to Styrofoam. Edit: It isn't really Styrofoam like cups, it is some sort of hard expanded foam. I will see how long the standard stuff lasts. Use a piece of grill cloth with plenty of overhang to glue to the backing. Grill cloth needs to be stretched very tight to keep it from fluttering. When I sprayed the glue, and let it setup for a few minutes, I had no problem stretching it and getting it down. I stretched it apart on one side and secured it. Then I kept pulling it tight as I went around till I was done.

This is what it looked like

Like I said, be sure to use enough overhang so when you stretch it and glue it, it has enough hold. I first cut the piece to fit then realized how little I left. That is why I glued the other piece on to help hold it. In fact, don't even worry about cutting to fit. Grill cloth stretches quite a bit. Just get it all down and then cut off what you want.

This is the finished product

And the real finished product

It isn't invisible, but it is indiscreet. Here is why I like this. Very simple. Since the cloth is glued on the underside, the majority of the bass will push against the backing, instead of pushing the cloth up if I did it on top of the whole deck. I can reach it from the underside for repairs if need be. Nothing is irreversible if I choose to do something different in the future. (Other than the big hole in my deck that is!) The cloth is nice and tight and there is no flutter with the sub pumpin'. I can't say how much of a difference it made because I did a bunch of Dynamating at the same time. Yes my sub performance has improved.